Self-assembly of C60 on a ZnTPP/Fe(001)–p(1 × 1)O substrate: observation of a quasi-freestanding C60 monolayer

Fullerene (C60) has been deposited in ultrahigh vacuum on top of a zinc tetraphenylporphyrin (ZnTPP) monolayer self-assembled on a Fe(001)–p(1 × 1)O substrate. The nanoscale morphology and the electronic properties of the C60/ZnTPP/Fe(001)–p(1 × 1)O heterostructure have been investigated by scanning tunneling microscopy/spectroscopy and ultraviolet photoemission spectroscopy. C60 nucleates compact and well-ordered hexagonal domains on top of the ZnTPP buffer layer, suggesting a high surface diffusivity of C60 and a weak coupling between the overlayer and the substrate. Accordingly, work function measurements reveal a negligible charge transfer at the C60/ZnTPP interface. Finally, the difference between the energy of the lowest unoccupied molecular orbital (LUMO) and that of the highest occupied molecular orbital (HOMO) measured on C60 is about 3.75 eV, a value remarkably higher than those found in fullerene films stabilized directly on metal surfaces. Our results unveil a model system that could be useful in applications in which a quasi-freestanding monolayer of C60 interfaced with a metallic electrode is required.


Introduction
Vertical heterostructures composed by organic molecules interfaced with metallic substrates have been the subject of intense experimental and theoretical investigation during the last two decades [1][2][3]. The interest in these hybrid systems has been boosted by their applications in new emerging fields, such as nanoscale catalysis [4,5], organic electronics [6,7], and spintronics [8,9], to name just a few. From a fundamental point of view, well-defined organic/inorganic heterostructures represent an interesting benchmark for the investigation of the boundary between materials possessing antithetic electronic and structural properties. In this frame, the molecule-metal interaction arising at the interface plays a crucial role in determining the morphology and the electronic properties of the hybrid organic/ inorganic system. With regard to the structural aspects, a compact molecular film is crucial to obtain high-performance devices, since an efficient charge carrier transport is hindered by morphological defects, such as grain boundaries or pinholes [10,11]. Moreover, crystalline and well-ordered layers are particularly suitable for spatially averaging measurements and for modeling by ab initio calculations. Periodic and compact films are generally obtained when the molecules possess enough surface mobility, that is, when the diffusion energy (E d ) is low compared to the thermal energy k B T, where T is the substrate temperature and k B is the Boltzmann constant [12]. Annealing the substrate during the film deposition could promote the growth of ordered layers even for high E d values (E d > k B T, with T = 300 K), but often the high annealing temperature required promotes the modification of the molecules or even their decomposition [13,14].
Another important aspect is the electronic coupling between the molecules and the metallic substrate. In this case, the key parameter is the adsorption energy (E a ), which is defined as the energy required to desorb a molecule from the surface. A high E a is characteristic of molecules chemisorbed on the substrate, where a relevant charge transfer between the overlayer and the substrate occurs. In contrast, a low E a is characteristic of physisorbed molecules, for which the adsorption is mediated by the weak van der Waals interaction with the substrate.
Chemisorption is the typical scenario for molecules stabilized on metallic substrates. Here, the hybridization between the molecular orbitals and the electronic states of the substrate generally modifies the intrinsic properties of the molecules, inducing the broadening of the molecular resonances, the narrowing of the band gap, and the development of interface states [15,16]. The ability to tailor the degree of electronic coupling between the molecules and the substrate is of utmost importance when it comes to embedding the interface in a specific application. For instance, if the molecules are interfaced with ferromagnetic electrodes in spin-valve architectures, the hybridization between the electronic states of the metallic substrate and the molecular orbitals is crucial to induce spin-polarized molecular states at the organic/inorganic interface [17][18][19]. Conversely, if either isolated molecules or self-assembled monolayers are adsorbed on solid surfaces for the investigation of their intrinsic properties, the minimization of the molecule-substrate interaction is desirable [20]. Furthermore, a weak molecule/metal electronic coupling is required in organic solar cells, because metallic states promote the relaxation of photo-excitations, lowering the cell efficiency [21].
It has been shown that a buffer layer interposed between the substrate and the molecular film can improve the crystallinity of the latter and reduce the electronic coupling with the support [22]. The buffer layer can either be a thin oxide film [23][24][25][26] or a single layer of 2D material, such as graphene [27,28], hexago-nal boron nitride [29][30][31] and MoS 2 [32,33]. Moreover, an organic layer inserted between the substrate and the overlayer has been shown to be effective in improving the order of the molecular film [34,35] or restoring its original electronic structure [36][37][38]. In this paper, we investigate the effects induced by a ZnTPP buffer layer covering the Fe(001)-p(1 × 1)O surface on the electronic and structural properties of a C 60 ultrathin film. The Fe(001)-p(1 × 1)O surface is characterized by a single layer of oxygen atoms, adsorbed in the hollow sites of the Fe(001) surface [39][40][41]. The deposition of a single layer of ZnTPP on Fe(001)-p(1 × 1)O leads to the stabilization of a well-ordered organic film, forming a (5 × 5) superstructure with respect to the substrate [42][43][44][45][46]. It is important to notice that the deposition of ZnTPP directly on the bare Fe(001) surface results in a completely disordered film [47], therefore the passivation of Fe(001) with oxygen is a crucial step to obtain a suitable molecular buffer layer. Since porphyrins molecules lie flat on the Fe(001)-p(1 × 1)O surface, the ZnTPP wetting layer provides an ideal buffer layer for the growth of C 60 , which forms a compact film weakly coupled with the metallic substrate.

Materials and Methods
The experiments were performed in two ultrahigh vacuum (UHV) systems. Clean Fe(001) is obtained by deposition of a thick Fe film (500 nm) by molecular beam epitaxy in UHV on a MgO(001) single crystal [48]. The Fe(001)-p(1 × 1)O surface was prepared by using the following procedure: the clean Fe substrate was exposed to 30 Langmuir of molecular oxygen at a pressure of = 2 × 10 −7 mbar and subsequently annealed at about 700 °C for 5 min. Porphyrins were sublimated by Knudsen effusion cells. The deposition flux was 0.5 ML/min, with 1 ML = 3.06 Å, as monitored by a quartz microbalance. C 60 was evaporated on top of 1 ML ZnTPP/Fe(001)-p(1 × 1)O. STM images have been acquired at room temperature in constant-current mode with custom-made electrochemically etched W tips.
Scanning tunneling spectroscopy (STS) data, that is, dI/dV curves for the investigation of the sample density of states (DOS), have been collected at room temperature, using a lockin amplifier with a modulation amplitude of 60 mV.
All STM and STS measurements have been carried out while keeping the sample grounded and applying a continuous or sinusoidal bias voltage to the tip. We followed the convention to indicate as positive the bias for which electrons tunnel from filled states of the tip to empty states of the sample.
The ultraviolet photoelectron spectroscopy (UPS) data have been acquired at normal emission with a 150 mm hemispher- ical electron analyzer from SPECS GmbH. The probing depth of UPS is a few angstroms [49]. A He lamp has been employed as a source of non-monochromatized unpolarized UV photons. The He-I line, with a photon energy of 21.2 eV, has been used to excite the sample. The full width at half maximum (FWHM) energy resolution of the UPS experiment is 0.05 eV.  that the deposition has been performed with the substrate kept at room temperature, we can estimate that E d for C 60 diffusing on ZnTPP is significantly lower than 25 meV. It is worth to notice that the ZnTPP buffer layer remarkably decreases E d with respect to the case of C 60 deposited at room temperature directly on either the Fe(001) or Fe(001)-p(1 × 1)O surfaces. In the former case, the diffusion of C 60 is completely hindered and fullerene forms a disordered film, while in the latter case a peculiar mode of growth, intermediate between diffusion-mediated and ballistic growth, is observed [23,50]. Figure 2b shows a blowup of one fullerene domain, where individual C 60 molecules are visible inside a hexagonal lattice with a lattice parameter of about 1 nm, a value very similar to that measured in C 60 films stabilized on either metallic [51] or oxide [25] substrates. This feature almost completely vanishes as soon as 1 ML of ZnTPP is deposited, indicating that oxygen remains buried at the ZnTPP/Fe(001)-p(1 × 1)O interface. In the 1 ML ZnTPP spectrum in Figure 3, the UPS peaks related to the main mole- cule ring and to the phenyl groups are labeled "R" and "Ph" [52,53], respectively, according to theoretical simulations performed on metal tetraphenyl porphyrins and metal porphyrins [54]. When an additional single layer of C 60 is added to this system, new features appear. The photoemission signal from the underlying ZnTPP layer, albeit affected by the screening action of C 60 (implying a rather large surface sensitivity of the technique, as also shown in [55] on a similar system), is still detected in those spectral regions not superimposed to the new C 60 features. In particular, peaks "a" and "b" can be readily assigned to HOMO and HOMO−1 features and their energetic positions match with their equivalents when a very thick layer of C 60 is grown directly on Fe(001)-p(1 × 1)O (top spectrum). The feature labeled "c" in Figure 3 is due to C 2p electrons [56]. Therefore, it is present with only slight modifications both in ZnTPP/Fe(001)-p(1 × 1)O and C 60 /ZnTPP/Fe(001)-p(1 × 1)O samples.

Results and Discussion
In order to determine the HOMO-LUMO gap of the C 60 film, STS measurements have been acquired for both negative and positive bias to investigate the filled and empty electronic states, respectively. Figure 4 shows STS spectra acquired on the ZnTPP/Fe(001)-p(1 × 1)O surface (red) and on the C 60 /ZnTPP/ Fe(001)-p(1 × 1)O system (black). The STS measurements acquired on ZnTPP/Fe(001)-p(1 × 1)O are in excellent agreement with those published in [43]. The STS curve referring to C 60 /ZnTPP/Fe(001)-p(1 × 1)O has been obtained by averaging several spectra acquired on equivalent C 60 domains. We acquired also spectra in different locations of single C 60 molecules, but not significant differences with a well-defined trend  were observed. In the negative energy range (filled electronic states) a strong resonance centered at about −2.60 eV is present, which we attribute to HOMO states, in excellent agreement with UPS measurements (−2.56 eV). In the positive energy range (empty states) of the STS spectrum the LUMO peak is visible at 1.15 eV, resulting in an electronic gap equal to 3.75 eV.
Finally, work function measurements have been performed to evaluate the charge transfer between the different layers constituting the heterostructure. Generally, electron transfer from the substrate (overlayer) to the overlayer (substrate) induces an increase (decrease) of the work function with respect to the bare surface. For the work function measurements, the sample has been biased with a voltage of 10 V to detect the onset of the secondary electrons. The onset position is determined as the intersection of the zero-current line and the tangent to the rising Table 1: Electronic coupling of C 60 with the Fe(001)-p(1 × 1)O and ZnTPP/Fe(001)-p(1 × 1)O substrates. E is the energy gap measured by electronbased spectroscopic techniques. U = E − γ is the on-site Coulomb energy, where γ = 1.6 eV is the HOMO-LUMO energy difference at equilibrium. ΔU surf and ΔU sub are variations of U with respect to the value of isolated C 60 due to the topmost layer and the substrate, respectively. edge of the data. Figure 5a displays a typical UPS spectrum in an energy range straddling the high-binding-energy cutoff of the secondary electrons, which we exploit for the evaluation of the work function for 1 ML ZnTPP/Fe(001)-p(1 × 1)O. The 10 eV offset due to the bias applied to the system has already been accounted for. In Figure 5b, the evolution of the work function for the different samples is presented. Starting from the bare substrate, the work function is reduced by about 0.3 eV after the deposition of 1 ML of ZnTPP, in agreement with previous measurements [42]. Such a decrease has been ascribed to charge transfer from ZnTPP to the Fe(001)-p(1 × 1)O substrate. When 1 ML of C 60 is added, the variation of the work function is within the experimental error, indicating a negligible charge transfer on the surface region upon C 60 adsorption.
The electronic properties of C 60 adsorbed on ZnTPP deserve a deeper discussion. We recall that the difference between the energies of LUMO and HOMO orbitals of C 60 at equilibrium is about γ = 1.6 eV, as determined experimentally [57] and theoretically [58]. However, the difference between the electron affinity and the ionization potential measured on isolated C 60 (in the gas phase) is about E s = 4.95 eV [57], considerably higher than γ. This discrepancy is given by the fact that the ionization potential (electron affinity) is not simply the difference between the vacuum level and the HOMO (LUMO) energies of C 60 at equilibrium, because an extra energy is required to remove (add) an electron from (to) the neutral molecule. Therefore, the gap measured with electron-based spectroscopic experiments is E = γ + U, where U is the on-site Coulomb energy [57]. The U term accounts for the fact that, when occupied states are probed, an electron is removed from the molecule, therefore the measured spectrum is not representative of the neutral but of the positively charged molecule. Similarly, when unoccupied states are probed, an electron is injected in the molecule and the system is negatively charged. For isolated C 60 molecules, the charging energy is U s = E s − γ = 3.35 eV. In [57], Esper et al. measured γ by performing PES on C 60 films highly doped with K. In this case, the LUMO orbitals were completely filled, therefore the charging energy was the same when HOMO and LUMO states were probed and the difference between the LUMO and HOMO energies was independent from U.
Generally, when C 60 is adsorbed on a substrate, the U term is drastically reduced by the electrostatic screening provided by the metallic or molecular support. In the former case, when an electron is added or removed from C 60 , the charged molecule is screened by an opposite image charge underneath the metal surface, while, in the latter case, the screening is provided by electric dipoles induced on the organic substrate. In order to evaluate the coupling between C 60 and the substrate, it is useful to quantify the reduction of the electronic gap E (or equivalently of the U term) with respect to that of the isolated molecule. In the case of the (111) surface of face-centered cubic bulk C 60 , the measured electronic gap is E b = 3.50 eV [23]. Therefore, the charging energy is U b = 1.90 eV. Defining ΔU as the variation of the Coulomb energy with respect to isolated C 60 , in the case of bulk C 60 , it is found ΔU = U b − U s = −1.45 eV. Such a decrease of U can be ascribed to the polarization of the nine molecules surrounding each C 60 located at the surface, six belonging to the topmost layer and three to the second layer. By considering an equal contribution for each molecule, every C 60 provides a screening of about ΔU = −0.16 eV.
Starting from this observation, it is possible to evaluate the screening provided by the Fe(001)-p(1 × 1)O and ZnTPP/ Fe(001)-p(1 × 1)O substrates on the C 60 film (see Table 1). To this aim, we can assume that ΔU is the sum of two contributions, the first one due to the screening provided by six surrounding C 60 molecules (ΔU surf ) and the second one provided by the substrate (ΔU sub ). As for ΔU surf , we consider for each sample the same value as found in bulk C 60 (111), because C 60 forms a hexagonal lattice also on top of the other substrates.
In the case of C 60 /Fe(001)-p(1 × 1)O, it is found ΔU sub = −0.59 eV. Therefore, the oxygen-passivated Fe(001) surface provides a higher screening with respect to a fullerene substrate. In contrast, for the C 60 /ZnTPP/Fe(001)-p(1 × 1)O sample, it is found ΔU sub = −0.24 eV, indicating a very low screening induced by the porphyrin buffer layer, even with respect to that provided by a substrate of bulk C 60 .

Conclusion
In conclusion, the electronic and morphological properties of a single layer of C 60 deposited on a ZnTPP/Fe(001)-p(1 × 1)O substrate have been investigated. The ZnTPP buffer layer promotes the surface diffusion of C 60 and the growth of a crystalline film at room temperature. The large HOMO-LUMO gap and the negligible charge transfer at the interface indicate that C 60 is electronically decoupled from the substrate. The C 60 /ZnTPP/Fe(001)-p(1 × 1)O multilayer represents a paradigmatic system in which the electronic properties of a single layer of fullerene in close proximity to a metallic substrate are preserved.

Funding
This work has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Project SINFONIA, Grant 964396.